The production of oxirane compounds such as propylene oxide and its higher homologs is described in Kollar U.S. Pat. No. 3,351,635. In accordance with the Kollar process, the oxirane compound may be prepared by epoxidation of an olefinically unsaturated compound (for example, propylene) by use of organic hydroperoxide and a suitable metal catalyst, such as a molybdenum compound. Kollar teaches that activity of the metal catalyst disclosed therein for expoxidation of primary olefins is high and can lead to high selectivity of propylene to propylene oxide. These selectivities are obtained at high conversions of hydroperoxide (50% or higher) which conversion levels are important for commercial utilization of this technology. In accordance with the Kollar process, the epoxidation reaction proceeds under pressure in a liquid state, and accordingly, a liquid solution of the metal catalyst is desired.
In the preparation of these compounds, for example, molybdenum salts, for the aforementioned purpose, various techniques have been used, many of which have been found to be extremely difficult to carry out efficiently on a commercial scale, and hence expensive, particularly for preparing hydrocarbon soluble compositions containing a high molybdenum content. In addition, a number of the above-identified catalyst materials, for example, molybdenum acid salts prepared by reaction of a molybdenum compound with a carboxylic acid as disclosed in Becker U.S. Patent Re 30,642, suffer from various disadvantages including poor solubility in the reaction medium and low metal concentration.
In an effort to increase the concentration of molybdenum in the reaction medium in the catalytic epoxidation of olefins, preparations of epoxidation catalysts from comparatively expensive molybdenum metal have been reported in the prior art, for example, in Sheng et al U.S. Pat. Nos. 3,453,218 and 3,434,975. However, the use of low cost starting materials, such as ammonium molybdate, in the preparation of epoxidation catalyst solutions soluble in hydrocarbons has been hampered due to the slow rate of dissolution, precipitation of solids as a result of decomposition of the dissolved molybdenum species, and unsatisfactory low molybdenum concentration in these solutions. Accordingly, a number of preparations of organic-soluble molybdenum containing catalysts from a variety of oxygen containing molybdenum compounds have been reported in the prior art. In this connection, attention is directed to Bonetti U.S. Pat. No. 3,480,563 which discloses the preparation of such catalysts by reacting molybdenum trioxide with a monohydric primary saturated acyclic alcohol having from 4 to 22 carbon atoms in the molecule or with a mono- or polyalkylene glycol monoalkyl ether or mixtures thereof. An earlier patent to Price et al, U.S. Pat. No. 3,285,942 discloses the preparation of glycol molybdates of specified formula by reaction of an alpha- and beta-alkane diol of from 2 to 18 carbon atoms with molybdic acid or related molybdenum compounds in the presence of an organic nitrogen base. Maurin et al U.S. Pat. No. 3,822,321 describes the oxidation of olefins with a hydroperoxide using a molybdenum catalyst prepared by reaction of molybdenum containing compound, such as molybdic acid or salt, with a polyalcohol. Epoxidation of olefins by use of a molybdenum catalyst, prepared by reacting an oxygen-containing molybdenum compound with amine (or an amine N-oxide) and alkylene glycol is also described by Lines et al in U.S. Pat. No. 4,157,346. Hagstrom et al U.S. Pat. No. 3,991,090 and 4,009,122 disclose a method of preparing molybdenum compound by reaction of an oxygen containing molybdenum compound with a polyhydroxy compound having vicinal hydroxyl groups in the presence of a hydrohalic acid. French Pat. No. 1,550,166 discloses that molybdic acid esters, and especially glycol esters of molybdic acid, provide certain advantages over previously known catalysts to effect epoxidation employing organic hydroperoxides in reaction with olefinic compounds. Cavitt U.S. Pat. No. 4,046,783 discloses the use in olefin epoxidation reactions of an oxidized alkyl molybdate complex catalyst prepared by contacting an inorganic molybdenum compound with an aliphatic monohydric alcohol in the presence of a weak base to form a lower oligometric alkyl molybdate compound which is then oxidized to form an oxidized alkyl molybdate complex catalyst. Also, ammonium molybdate epoxidation catalyst solutions are described in U.S. Pat. Nos. 3,956,180 and 2,795,552.
When an ammonium molybdate is heated together with a hydrocarbon carboxylic acid, the ammonium molybdate reacts with the carboxylic acid to produce a molybdenum solution. However, this reaction requires a considerable length of time in order to produce a homogeneous molybdenum solution and, when the ratio of the ammonium molybdate to hydrocarbon carboxylic acid exceeds a certain level, the molybdenum precipitates within the reaction system after a prolonged period and as a result, a homogeneous solution containing a high concentration of molybdenum cannot be obtained.
Accordingly, it is an object of the present invention to provide for the production of novel molybdenum-containing salt catalyst compositions which are characterized by improved dissolution rates in organic hydrocarbon solutions, high molybenum concentrations in organic solutions, and provide stable dissolved molybdenum species free of precipitation of solids due to decomposition, thereby resulting in improved and increased catalyst preparation and productivity.
An additional object of the present invention is to provide a process for the epoxidation of olefinic compounds by use of the hydrocarbon-soluble molybdenum salt catalyst compositions of the invention, thereby resulting in increased yield and selectivity to desired alkylene oxide, e.g. propylene oxide, in the epoxidation of a primary olefin, e.g. propylene, while at the same time reducing production of undesired by-products.